Methods for transitioning into reduced braking performance modes upon failure of a primary braking system

ABSTRACT

A method according to the present disclosure includes identifying a failure in functionality of the primary braking system, and upon identifying the failure, mitigating an abrupt increase in a pedal travel distance required to brake or otherwise decelerate the vehicle so as to provide a smooth transition from a primary braking mode to a fallback braking mode. Mitigating the increase in the pedal travel distance may include initiating a transition to the fallback braking mode, activating at least one of a plurality of transition braking modes, and gradually increasing the pedal travel distance by deactivating at least one of the at least one previously activated transition braking modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/223,637, filed on Jul. 29, 2016, the contents of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

Aspects of the present disclosure relate generally to methods fortransitioning into reduced braking performance modes upon failure of aprimary braking system. More particularly, aspects of the presentdisclosure relate to methods for providing a smooth transition into afallback braking mode upon failure of a primary braking system of anautomotive vehicle.

BACKGROUND

Brake systems for a vehicle often rely on hydraulic fluid to transferforce into torque at the wheels of the vehicle. The force transmittedvia hydraulic fluid may be generated directly from pressure imparted ona pedal by a driver's foot. Alternatively, the force transmitted viahydraulic fluid may be generated from, and/or multiplied by, asemi-connected or fully-connected actuator. The actuator may bemechanical (e.g., a vacuum booster) or electrical (e.g., an electricmotor).

Upon failure of the hydraulic fluid transfer system (e.g., leak or valvemalfunction) or failure of the force generation and/or multiplicationmeans, brake systems can be designed to have a fallback braking mode.Generally, a fallback braking mode is fully actuated by the driver'sfoot to apply counter-propulsive torque at the wheels of the vehicle.

From the driver's perspective, fallback braking modes generally feelvastly different than the primary braking mode of the vehicle. Primarybraking modes call for shorter pedal travel and lower input force thanwhat is generally required in a fallback mode. Primary braking modesalso provide a higher vehicle deceleration than fallback modes generallyoffer. In a fallback mode, a brake pedal has a longer travel and a“spongy” feel for the driver such that pedal depression can seem to haveno impact on the deceleration of the vehicle until considerable pedaltravel distance and force have been applied. Thus, in the event of aprimary brake system failure, the sudden change to a fallback moderesults in a potentially frightening situation for the driver as thebrake pedal assumes unfamiliar performance characteristics. A driver mayinterpret the change to a fallback mode as a complete loss of brakingpower and as a result may not push far and/or hard enough on the brakepedal to impart any counter-propulsive torque whatsoever.

Therefore, it may desirable to provide a less abrupt transition to afull fallback braking mode in the event of failure of the primarybraking system. Further, it may be desirable to transition to at leastone intermediate backup braking mode that has a shorter pedal travel andprovides a stiffer pedal feel than a final, full fallback braking mode.Further, it may be desirable to provide a smooth and/or multi-phasetransition to a final fallback braking mode in the event of failure ofthe primary braking system.

SUMMARY

Exemplary embodiments of the present disclosure may solve one or more ofthe above-mentioned problems and/or may demonstrate one or more of theabove-mentioned desirable features. Other features and/or advantages maybecome apparent from the description that follows.

In accordance with various exemplary embodiments of the presentdisclosure, a method for transitioning a braking system of a vehiclebetween primary and fallback modes is provided. The method includesidentifying a failure in functionality of the primary braking system,and, upon identifying the failure, mitigating an abrupt increase in apedal travel distance required to brake or otherwise decelerate thevehicle so as to provide a smooth transition from a primary braking modeto a fallback braking mode. The mitigating the increase in the pedaltravel distance includes initiating a transition to the fallback brakingmode, activating at least one of a plurality of transition brakingmodes, and gradually increasing the pedal travel distance bydeactivating at least one of the at least one previously activatedtransition braking modes.

Additional objects and advantages will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the present disclosureand/or claims. At least some of these objects and advantages of thepresent disclosure may be realized and attained by means of the elementsand combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the claimed subject matter; rather the claimsshould be entitled to their full breadth of scope, includingequivalents. The accompanying drawings, which are incorporated in andconstitute part of this specification, illustrate exemplary embodimentsof the present disclosure and together with the description, serve toexplain principles of the present teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

At least some features and advantages of the present teachings will beapparent from the following detailed description of exemplaryembodiments consistent therewith, which description should be consideredwith reference to the accompanying drawings, wherein:

FIG. 1 is a schematic bottom view of a vehicle, according to anexemplary embodiment of the present teachings;

FIG. 2 is a schematic bottom view of a vehicle when a primary brakingsystem is partially functional and a transition to a fallback brakingmode has been initiated, according to an exemplary embodiment of thepresent teachings;

FIG. 3 is a flow chart depicting a method of determining a functionalstate of a braking system, according to an exemplary embodiment of thepresent teachings;

FIG. 4 is a flow chart depicting a first exemplary method oftransitioning to a fallback braking mode, according to an exemplaryembodiment of the present teachings;

FIG. 5 is a flow chart depicting a second exemplary method oftransitioning to a fallback braking mode, according to another exemplaryembodiment of the present teachings; and

FIG. 6 is a flow chart depicting a third exemplary method oftransitioning to a fallback braking mode, according to yet anotherexemplary embodiment of the present teachings.

Although the following detailed description makes reference to exemplaryillustrative embodiments, many alternatives, modifications, andvariations thereof will be apparent to those skilled in the art.Accordingly, it is intended that the claimed subject matter be viewedbroadly.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to various exemplary embodiments,examples of which are illustrated in the accompanying drawings. Thevarious exemplary embodiments are not intended to limit the disclosure.To the contrary, the disclosure is intended to cover alternatives,modifications, and equivalents of the exemplary embodiments. In thedrawings and the description, similar elements are provided with similarreference numerals. It is to be noted that the features explainedindividually in the description can be mutually combined in anytechnically expedient manner and disclose additional embodiments of thepresent disclosure. Furthermore, elements and their associated featuresthat are described in detail with reference to one embodiment may,whenever practical, be included in other embodiments in which they arenot specifically shown or described. For example, if an element isdescribed in detail with reference to one embodiment and is notdescribed with reference to a second embodiment, the element maynevertheless be claimed as included in the second embodiment.

The present disclosure contemplates various methods for transitioningbetween a primary braking mode and fallback braking modes (i.e., reducedbraking performance modes upon a failure within a primary brakingsystem). In accordance with the present disclosure, a braking system ofa vehicle may include a primary braking system and a backup brakingsystem. The backup braking system may be configured to select a backupbraking mode based on a functional state of the primary braking system,the motion of the vehicle, brake pedal travel, and/or the duration oftime that has passed since the failure within the primary system wasinitially detected. The primary braking system may be, for example, abrake-by-wire system or a mechanical push-through braking system.

The braking system also may include a parking brake system. The parkingbrake system may be, for example, a mechanical parking brake system oran electronic parking brake system. A parking brake system may beconfigured to be actuated in various manners, for example, via a simpleswitch (e.g., hand lever) or via an electronic switch (e.g., electronichand lever) operatively connected to a sensor such as an angle sensor.The parking brake system may supplement the primary braking systemand/or the backup braking system, as described herein.

Various embodiments of a method for transitioning a braking systembetween primary and fallback modes in accordance with the presentdisclosure can include reducing a working volume of a primary brakingsystem so that hydraulic braking energy is supplied to wheels of asingle axle of the vehicle, and selecting a source of braking energy tobe supplied to wheels of another axle of the vehicle by an electronicparking brake. A speed of the vehicle may provide the basis fordetermining the source of braking energy to be supplied to wheels ofanother axle of the vehicle by an electronic parking brake. When adetermined speed of the vehicle is above a threshold speed, a source ofbraking energy to be supplied to wheels of another axle of the vehicleby an electronic parking brake may include supplying braking energy viaantilock braking. In the event that a determined speed of the vehicle isbelow a threshold speed, a source of braking energy to be supplied towheels of another axle of the vehicle by an electronic parking brake mayinclude supplying braking energy via dynamic clamping in a slipdependent manner.

In addition to supplemental braking provided by a parking brake system,in various embodiments of a method for transitioning a braking systembetween primary and fallback modes in accordance with the presentdisclosure, other sources of counter-propulsive torque may be engaged,such as, for example, engine braking, transmission braking, addingengine loads, and/or shifting the vehicle into reverse or park. asdescribed herein. In the event of a failure of the primary brakingsystem, by selectively modifying the primary braking system andactivating various sources of braking/counter propulsive torquedescribed herein, a smooth transition to a full fallback braking modecan be achieved. A driver of a vehicle may observe such a smoothertransition by experiencing a stiffer pedal feel throughout thetransition to a final, full fallback braking mode.

Turning to FIG. 1, a schematic bottom view of an exemplary embodiment ofa vehicle 10 is shown, which includes a left-front wheel 12, aright-front wheel 14, a left-rear wheel 16, and a right-rear wheel 18.As depicted in the exemplary embodiment of FIG. 1, vehicle 10 may be anautomobile, such as, for example, a passenger car. However, the variousexemplary embodiments described herein may be used in other types ofautomobiles and other types of vehicles familiar to one skilled in theart, such as, for example, work vehicles, construction vehicles, andother vehicles one skilled in the art is familiar with.

Vehicle 10 may include a primary braking system 30 to slow and/or stoprotation of wheels 12, 14, 16, and 18. According to an exemplaryembodiment, primary braking system may be a brake-by-wire braking systemthat communicates with electronically controlled valves to supplyhydraulic fluid to braking devices associated with wheels 12, 14, 16,and 18. For example, primary braking system 30 may include a controller20 and a pressure source 22 in communication with wheel inlet valves 13,15, 17, and 19, respectively, which are configured to supply hydraulicfluid to braking devices (not shown) of wheels 12, 14, 16, and 18.Controller 20 may be in communication with brake pedal 50 such that theforce applied to the brake pedal 50 and/or the distance traveled by thebrake pedal 50 upon depression by a driver is detected and/or receivedby the controller 20 via a mechanical and/or electrical connection. Thecontroller 20 may use the input(s) communicated from the brake pedal todictate the function of at least the pressure source 22 and the wheelinlet valves 13, 15, 17, and 19. Pressure source 22 may be, for example,an electric brake booster, vacuum booster, a pressure reservoir, apiston-cylinder, a pump, or other device familiar to one of ordinaryskill in the art for providing pressure for a braking system. Anelectric brake boost device compatible with the methods of the presentdisclosure may include a ball screw mechanism to move a piston in achamber. Wheel inlet valves 13, 15, 17, and 19 may be dynamicallycontrollable and a part of an anti-lock braking (ABS) system and/orelectronic speed control (ESC) system, therefore, the valves 13, 15, 17,and 19 may be incorporated into methods for transitioning into afallback braking mode of the present disclosure without incurringsignificant additional cost or complexity to the overall braking systemof the vehicle 10 because the vehicle 10 already includes an ABS and/orESC system.

According to an exemplary embodiment, the primary braking system 30 mayinclude valves in communication with brake lines in communication withwheel inlet valves 13, 15, 17, and 19. For example, the primary brakingsystem 30 may include a first primary pressure supply valve 24 incommunication with a first brake line 52 communicating with wheel inletvalve 13 for left-front wheel 12 and wheel inlet valve 15 forright-front wheel 14, and a second primary pressure supply valve 26 incommunication with a second brake line 54 communicating with wheel inletvalve 17 for left-rear wheel 16 and wheel inlet valve 19 for right-rearwheel 18, such as when brake lines 52, 54 have a front-rear splitconfiguration, as shown in the exemplary embodiment of FIG. 1. However,brake lines 52, 54 are not limited to a front-rear split configurationand may have other configurations, such as, for example, adiagonal-split (i.e., one brake line is in communication with theleft-front wheel and the right-rear wheel, and the other brake line isin communication with the right-front wheel and the left-rear wheel) orother configurations that one skilled in the art is familiar with.

Braking devices (not shown) of wheels 12, 14, 16, and 18 may include,for example, a brake caliper with a piston that engages a brake padagainst a rotor of a wheel, according to an exemplary embodiment.Alternatively, according to various other exemplary embodiments, brakingdevices (not shown) of wheels 12, 14, 16, and 18 may include a wedgebrake system that includes a brake pad connected to a wedge, the brakepad being configured to be pushed between a brake caliper and a brakerotor via electronic motor(s). In yet other alternative exemplaryembodiments, braking devices (not shown) of wheels 12, 14, 16, and 18may be drum brakes, In still other alternative exemplary embodiments,braking devices (not shown) of wheels 12, 14, 16, and 18 may be devicesconfigured to apply counter-propulsive force via means other thanfriction, such as, for example, regenerative braking via electric motorsthat deliver stored chemical energy or regenerative braking viaflywheels that deliver stored kinetic energy. The braking system mayinclude other components not shown in FIG. 1, which may have beenomitted for clarity. For example, overall braking system may includeother components one skilled in the art would be familiar with, such as,for example, pumps, pressure reservoirs, hydraulic pistons, brake outletvalves, and fluid returns.

When performance of pressure source 22 is reduced (e.g., the ability toboost brakes is reduced or lost), a transition to a fallback brakingmode may be initiated. A sensor (not shown) may detect when thereduction in braking performance capability occurs and trigger theinitiation of a transition to a fallback braking mode. In otherexemplary embodiments, a sensor detects the loss of ability to boostbrakes during the first depression of the brake pedal 50 after a brakeboosting failure occurs, thus the trigger occurs once the brake pedal 50has been at least partially depressed by the driver. A sensor in thistype of system may be an electric motor sensor that is configured todetect a lack of motor movement after a target pressure is requested inaccordance with a pedal depression. Also, two sensors may be configuredto detect when pedal depression or other driver movement createspressure in one chamber of the brake system, but the generation ofpressure in accordance with a subsequent request for pressure fails tooccur in another chamber of the brake system. Furthermore, in variousexemplary embodiments according to the present disclosure, a sensor maydetect when a current draw of a phase current or field-effect transistor(FET) current is outside of a range corresponding to the expectedcurrent draw of the level of request received, from which a stuckactuator may be inferred.

In some other exemplary embodiments, the sensor may be capable ofcontinuously monitoring the pressure source 22 such that the transitionis initiated nearly instantaneously upon the loss of brake boostingability and before any pressure is applied to the brake pedal 50 by thedriver. A sensor in this type of system may, for example, be a brakefluid switch that detects when the hydraulic fluid reservoir is drainedor a sensor that detects when a complete loss of communication with abrake module occurs, which can indicate complete failure of the primarybrake system. In these scenarios, If the controller for detecting driverinput(s) is internal to the brake module that has failed, theindications representing driver input(s) (e.g., pedal travel) will nothave high resolution. In event of this, the output of a brake on-off(BOO) switch, generally used for cruise control cancellation and stoplamp activation, can be used as a switch that also initiates thetransition to a fallback braking mode according to the presentdisclosure. Upon receiving an indication that brake boosting ability hasbeen lost, initiation of a transition to a fallback braking mode mayoccur or, confirmation that the braking is not actually occurring may bemade prior to such initiation by looking to the output of a longitudinalacceleration sensor to observe whether an appreciable increase indeceleration coinciding with the activation of the BOO switch has or hasnot occurred.

When fully functional, the controller 20 may direct the primary brakingsystem 30 to supply hydraulic fluid to braking devices of all wheels ofthe vehicle. In the exemplary embodiment of FIG. 1, primary pressuresupply valves 24, 26 and wheel inlet vales 13, 15, 17, and 19 are shownin an open state to represent a fully functional state of the primarybraking system 30.

Upon the detection of a fault in the primary braking system 30, forexample a brake booster failure as described above, controller 20 mayclose valves in order to reduce the working volume of the overall brakesystem. Turning to FIG. 2, a schematic bottom view of an exemplaryembodiment of the vehicle 10 is shown with an exemplary configuration ofthe braking system in an initial state after the ability to boost brakeshas been lost and the transition to a fallback braking mode has beeninitiated. Controller 20 may have control over primary pressure supplyvalves 24 and 26, and including wheel inlet valves 13, 15, 17, and 19.Thus, upon detection of a brake booster failure, the controller 20 may,for example, close primary supply valve 26 and rear wheel inlet valves17 and 19 so that all of the fluid pressure that the primary brakingsystem is able to generate while the brake booster is non-functionalgoes to the braking devices for the front wheels 12 and 14. Reducing theworking volume of the overall brake system in this way at an initialstage of the transition to a fallback braking mode provides brake pedal50 with a shortened pedal travel and a stiffer responsiveness at lowpedal travel distances (i.e., a higher force-to-travel relationship).Thus, at the initial stage of the transition to a fallback braking mode,the pedal feel of the brake pedal 50 can be maintained closer to thepedal feel of the brake pedal 50 during a normal braking mode (i.e.,when the primary braking system is fully functional). Alternative to theabove (but not shown), the foundation brakes of the rear wheels 16 and18 may be isolated from receiving hydraulic fluid pressure by closingonly the wheel inlet valves 17 and 19 or by only closing the primarysupply valve 26.

Although the exemplary embodiment of FIG. 2 depicts the braking systembeing configured to provide hydraulic braking fluid pressure to thebraking devices of front wheels 12 and 14 while isolating the brakingdevices of rear wheels 16 and 18 in the initial stage of the transitionto a fallback braking mode, the braking system is not limited to thisconfiguration. For example, the braking system may provide hydraulicbraking fluid to the braking devices of the rear wheels 16 and 18 whileisolating the braking devices of the front wheels 12 and 14 in theinitial stage of the transition to a fallback braking mode.

To provide additional braking energy in the event that a transition to afallback braking mode is initiated by a detected failure of the primarybraking system, a parking brake system 60 of vehicle 10 may be used tobrake one or more wheels of vehicle 10, as shown in FIGS. 1 and 2.Parking brake system 60 may be, for example, an electric parking brake,according to an exemplary embodiment. As shown in the exemplaryembodiment of FIG. 2, parking brake system 60 may communicate withparking brake devices 62 and 64 of rear wheels 16 and 18 that applybraking energy to rear wheels 16 and 18 to slow and/or stop rotation ofrear wheels 16 and 18. As a result, parking brake system 60 may be usedto provide additional braking energy as part of a transition to afallback braking mode without incurring significant additional cost orcomplexity to the braking system of vehicle 10 because vehicle 10already includes parking brake 60.

According to an exemplary embodiment, when controller 20 closes primarypressure supply valve 26 and wheel inlet valves 17 and 19 for rearwheels 16 and 18 so that brake fluid pressure is only supplied tofoundation brakes of front wheels 12 and 14, parking brake system 60 maybe used to provide additional braking energy, as shown in the exemplaryembodiment of FIG. 2. Parking brake system 60 may be used to slow and/orstop rear wheels 16 and 18. In other words, the hydraulic pressure frompressure source 22 may be directed to only the wheels of a single axle,such as, for example, a front axle, while a parking brake appliesbraking energy to the wheels of another single axle, such as, forexample a rear axle. Thus, an initial stage of a transition to afallback braking mode may provide a pedal feel of the brake pedal 50that is closer to the pedal feel of the brake pedal 50 during a normalbraking mode (i.e., when the primary braking system is fully functional)and provide a driver with a more confident feeling regarding braking.The absence of hydraulic pressure applied to rear wheels 16 and 18 mayalso allow parking brake 60 to operate with a higher amount of torquebecause the risk of vehicle instability may be minimized by using awheel-slip control system.

In one exemplary embodiment, the initial stage of the transition to afallback braking mode involves simultaneously supplying hydraulic fluidpressure to the braking devices of the front wheels 12, 14 and actuatingthe parking brake to only apply braking energy to the rear wheels 16,18. In another exemplary embodiment, the initial stage of the transitionto a fallback braking mode involves supplying the hydraulic fluidpressure to the braking devices of the front wheels 12, 14, waiting aperiod of time, and then actuating the parking brake to apply brakingenergy to the rear wheels 16, 18. For example, actuation of the parkingbrake assistance may be delayed until the amount of force applied to thebrake pedal 50 by the driver, as measured by a sensor in communicationwith the brake controller 20, reaches a threshold amount of force.Delaying application of the parking brake energy to the rear wheels isan accommodation for the facts that EPBs have higher efficacy relativeto the limited pressure being provided to the front wheels and thatcurrent EPBs have relatively unrefined and unresponsive control.Accordingly, the incorporation of a delay is to allow for sufficientbraking to occur at the front wheels 12, 14 before applying parkingbrake energy to the rear wheels 16, 18 so that any vehicle instabilitythat may occur upon application of the parking brake can be diminished.In another exemplary embodiment, actuation of the parking brakeassistance may be delayed until the speed of the vehicle 10 is reducedto a specific threshold via other means for decelerating the vehicle.

According to an exemplary embodiment, parking brake system 60 may be incommunication with and controlled by brake controller 20 (e.g., anintegrated electronic parking brake (iEPB)), although parking brakesystem 60 may be instead controlled by a separate controller incommunication with brake controller 20. If controlled by a separatecontroller, the brake on-off (BOO) switch can trigger actuation of theparking brake system in the event of a failure within the brakecontroller 20. Further, parking brake devices 62 and 64 may be, forexample, brake devices actuated by electric motors. For example, parkingbrake devices 62 and 64 may be screw devices to apply pressure to brakecalipers of vehicle wheels, with the screw devices being actuated byelectric motors driven by an electric current controlled by controller20.

Although the exemplary embodiment of FIG. 2 depicts the braking systembeing configured to provide hydraulic braking fluid pressure to thefoundation brakes of front wheels 12, 14 and braking energy from parkingbrake system 60 to rear wheels 16 and 18 in the initial stage of thetransition to a fallback braking mode, the braking system is not limitedto this configuration. For example, parking brake system 60 may providebraking energy to front wheels 12 and 14 and hydraulic braking fluidpressure may be applied to the foundation brakes of rear wheels 16 and18 in the initial stage of the transition to a fallback braking mode.

Parking brake systems of the exemplary embodiments described herein,such as parking brake systems 60 of the exemplary embodiments of FIGS. 1and 2, may utilize proportional braking. Thus, instead of using asingle, predetermined braking force to slow and/or stop the wheels of avehicle, the braking force is proportional to the amount of pedaldepression made by a driver to a pedal. As a result, a driver may feelthat the vehicle is braking in a manner corresponding to the amount ofpedal depression, even when braking is achieved by a braking mode usingthe parking brake in a proportional manner.

According to an exemplary embodiment, when a parking brake system is anelectric parking brake, the parking brake system may receive use signalsfrom a brake pedal and/or sensors to determine the amount of pedaldepression by a driver and use the signals to control the parking brakeproportionally to the amount of pedal depression. In the exemplaryembodiments of FIG. 2, brake controller 20 may receive signals from thebrake pedal and/or sensors to determine the amount of pedal depressionand then control parking brake devices 62, 64 in a proportional manner.For example, when parking brake devices 62, 64 are devices actuated byelectric motors, such as screw devices to apply pressure to a brakecaliper of a wheel, brake controller 20 may control the amount ofcurrent supplied to the electric motors actuating parking brake devices62, 64 so the parking brake applies braking energy in a mannerproportional to the amount of pedal depression. Whether or not currentis supplied to the electric motors actuating parking brake devices 62,64 may be determined based on whether a threshold for the current drawnto the electric motors actuating has been eclipsed (if eclipsed,application of the EPB can be ceased or reduced). The threshold currentdraw may correspond to the amount of current that is drawn when the EPBis fully applied and the electric motor is stalled or near stalling. Inanother exemplary embodiment, the parking brake energy may be applied inproportion to the duration of the application of the parking brake.Moreover, a parking brake system may be configured to apply brakingenergy at multiple stages according to position pre-programmed in theelectric motors that actuate the parking brake devices 62, 64.

According to an exemplary embodiment, a parking brake system may becontrolled to utilize anti-lock braking (ABS). For example, when parkingbrake system 60 of the exemplary embodiments of FIGS. 1 and 2 are usedin a transition to a fallback braking mode, brake controller 20 mayreceive signals from wheel speed sensors, such as wheel speed sensorsfor rear wheels 16, 18 to indicate whether a wheel has locked. When awheel speed signal indicates that a wheel has locked, brake controller20 may control parking brake devices 62, 64 to apply less force, such asby reducing a current supplied to motors actuating parking brake devices62, 64, so that a wheel unlocks. According to an exemplary embodiment, apark brake system may be controlled to use ABS when also implementingproportional braking, as described above.

As described above, when a primary braking system becomes either fullyor partially non-functional, a transition to a fallback braking mode maybe initiated to slow and/or stop rotation of the wheels of the vehicle.A primary braking system may become fully non-functional (i.e., theability to provide any hydraulic fluid pressure to the braking devicesis lost completely, for example, because of a brake controller failureor a complete loss of hydraulic braking fluid) or only partiallyfunctional (i.e., the ability to provide hydraulic fluid pressure to thebraking devices is reduced, for example, because one or more aspects ofthe primary braking system, such as a pressure source's ability to boostthe braking force, has failed). In view of this, an overall brakingsystem may be configured to selectively actuate different transitionalbraking modes prior to selecting a fallback braking mode according tothe functional state of the primary braking system so that thetransition may be efficiently conducted.

In accordance with an exemplary embodiment of the present teachings, acontroller of a braking system of vehicle may be configured to select atransitional braking mode prior to selecting a fallback braking modebased on the functional state of the primary braking system. Forexample, brake controller 20 of primary braking system may function as acontroller to determine if the primary braking system is partiallyfunctional and determine a transitional braking mode prior to bringingthe braking system into a final fallback braking mode. Controller 20 maybe in communication with pressure source 22, primary pressure supplyvalves 24 and 26, and wheel inlet valves 13, 15, 17, 19 so thatcontroller 20 may be able to determine the functional status of thesecomponents. For example, controller 20 may receive signals from pressuresource 22, primary pressure supply valves 24 and 26, and/or wheel inletvalves 13, 15, 17, 19 indicating the functional status of eachcomponent. In another example, the absence of a signal from pressuresource 22, primary pressure supply valves 24 and 26, and/or wheel inletvalves 13, 15, 17, 19 can indicate the functional status of eachcomponent (e.g., that the component is non-functional). Afterdetermining the functional status of pressure source 22, primarypressure supply valves 24 and/or 26, and/or wheel inlet valves 13, 15,17, 19, controller 21 may select a functional state of the overallbraking system when primary braking system is at least partiallyfunctional.

If the various components are all functional, controller 20 may controlprimary braking system in a normal state, such as without the initiationof any of the transitional braking modes. Otherwise, if primary brakingsystem is either partially or fully non-functional, a transitionalbraking mode prior to a fallback braking mode may be initiated by thecontroller 20.

Turning to FIG. 3, a braking method 300 is shown for an exemplaryembodiment in which the functional state of a primary braking system isdetermined. In a first step 310, information may be received about thefunctional state of a primary braking system. For example, a controllermay monitor a functional status of the braking system of vehicle 10 inFIGS. 1 and 2, including pressure source 22, primary pressure supplyvalves 24 and 26, and/or wheel inlet valves 13, 15, 17, 19. In step 320,the functional state of the primary braking system 30 may be determinedbased on the information received in step 310, such as to determinewhether primary braking system 30 is fully functional, partiallyfunctional, or fully non-functional.

If the primary braking system is fully functional, the method mayproceed to step 330, in which the braking system is used according tonormal operating conditions, for example, as represented in exemplaryFIG. 1. The method may proceed from step 330 to step 310 to repeat theoverall braking method 300 on a periodic basis. If it is determined instep 320 that the primary braking system is fully non-functional, themethod 300 proceeds to step 350, in which a transition to a fallbackbraking mode occurs according to the exemplary embodiments describedherein in which the primary braking system is fully non-functional. Ifit is determined in step 320 that the primary braking system ispartially functional, the method 300 proceeds to step 340, in which atransition to a fallback braking mode occurs according to the exemplaryembodiments described herein in which the primary braking system ispartially functional.

As discussed above, when the primary braking system 30 is partiallyfunctional (e.g., the ability to boost brakes is lost), proceeding fromstep 340 of method 300, the controller 20 may initiate, a transition toa fallback braking mode in which the working volume of the brakingsystem is reduced such that hydraulic fluid pressure is supplied tofewer than all of the wheels of vehicle 10. For example, controller 20may supply hydraulic fluid to braking devices of only two wheels ofvehicle 10 (e.g., wheels of a single axle), such as front wheels 12 and14, but not rear wheels 16 and 18. Thus, the braking power of thehydraulic fluid pressure is focused on front wheels 12 and 14. Parkingbrake system 60 may be used to brake rear wheels 16 and 18.

Turning to FIG. 4, an exemplary method 400 to transition to a fallbackbraking mode once the primary braking system has been determined to bepartially functional at step 340 of method 300 is shown. At step 405,the brake controller may close rear wheel inlet valves and/or a primarypressure supply valve to reduce the working volume of the hydraulicbraking system such that hydraulic fluid pressure is supplied to only tothe front wheels of the vehicle. At step 410, which can optionally occurconcurrently with step 405, information may be received about the amountof force being applied to the brake pedal by the driver. For example,force information can be detected and/or approximated by a pedal travelsensor, a pedal angle sensor, an ABS unit pressure sensor, a two-stagebrake on-off switch, or any other method for sensing a driver's intentfor braking, and then communicated to the controller. In step 415, theamount of force being applied to the brake pedal by the driver may bedetermined based on the information received in step 410 in order todetermine whether the parking brake system 60 should be actuated tosupply braking energy to the rear wheels.

If the amount of force being applied to the brake pedal by the driver isdetermined to be below a specified threshold level of force, the methodmay proceed to step 420, in which the parking brake system is notactuated to supply braking energy to the rear wheels of the vehicle.Then the method may proceed from step 420 to step 410 to repeatedlydetermine whether to actuate the parking brake system to supply brakingenergy to the rear wheels on a periodic basis. If the amount of forcebeing applied to the brake pedal by the driver is determined to meet orexceed a specified threshold level of force, the method may proceed tostep 425, in which actuation of the parking brake system to supplybraking energy to the rear wheels of the vehicle is begun. To beginactuation of the parking brake system to supply braking energy to therear wheels, the method may proceed to step 430, in which informationmay be received regarding the speed of the vehicle. For example, speedinformation can be detected and/or approximated by a speedometer andthen communicated to the controller. In step 435, how to actuate theparking brake to supply braking energy to the rear wheels may bedetermined based on the information received in step 430.

Where, as described above, the parking brake system is an electronicparking brake system, and vehicle speed is determined to be above aspecified threshold level of speed, the method may proceed to step 440,in which the electronic parking brake is actuated and controlled to useABS while supplying braking energy to the rear wheels as describedabove. An electronic parking brake actuated and controlled to use ABScycles to a pre-defined load and then releases the load (regardless ofwhether wheel slip has occurred or not). The duty cycle of the on/offapplication of the load may be varied based on the force being appliedto the brake pedal and/or vehicle speed. The method may proceed fromstep 440 to step 430 to repeatedly determine how to actuate the parkingbrake to supply braking energy to the rear wheels. If vehicle speed isdetermined to be at or below a specified threshold level of speed, themethod may proceed to step 445, in which electronic parking brake isactuated and supplies braking energy to the rear wheels via dynamicclamping in a slip dependent manner. A slip dependent manner means thatwhen the electronic parking brake is clamped, it will engage the wheelup to a rear slip threshold where wheel slip occurs, then release thewheel. The rear slip threshold may vary, for example, according to thepercentage of deviation from vehicle speed or an amount of reduction perunit time relative to previous wheel rotation speeds.

Steps 410 through 445 can occur concurrently with or after the workingvolume of the hydraulic braking system is reduced such that hydraulicfluid pressure is supplied to only to the front wheels of the vehicle atstep 405. Once the working volume of the hydraulic braking system hasbeen reduced at step 405 and the electronic parking brake has beenactuated to supply braking energy to the rear wheels via dynamicclamping in a slip dependent manner at step 445, the method may proceedto step 450, in which information may be received about whether or notthe vehicle is at a standstill. For example, vehicle speed and/orstandstill information can be detected or approximated by a speedometer,GPS, a sensor operably connected to the power train (e.g., in thetransmission or electric motor drive), or any other method for sensing avehicle's motion, and then communicated to the brake controller. In step455, it may be determined whether or not the vehicle is at a standstillbased on the information received in step 450 in order to bring thebraking system into the final fallback braking mode.

If it is determined in step 455 that the vehicle is not at a standstill,the method 400 proceeds to step 460, in which the working volume of thehydraulic braking system is maintained at a reduced volume and theelectronic parking brake remains actuated to supply braking energy tothe rear wheels via dynamic clamping in a slip dependent manner. Themethod may proceed from step 460 to step 450 to repeatedly determinewhether to bring the braking system into the final fallback braking modeon a periodic basis. If it is determined that the vehicle is at astandstill, the method may proceed to step 465, in which the brakingsystem is brought into the final fallback braking mode. To bring thesystem into the fallback braking mode, the controller may reopen therear wheel inlet valves and/or a primary pressure supply valve in orderto increase the working volume of the hydraulic braking system such thathydraulic fluid pressure is supplied to all of the wheels of the vehicle(i.e., both the front and the rear wheels). Also, in bringing the systeminto the fallback braking mode, the parking brake may be removed fromengagement with the rear wheels. After removing the parking brake fromengagement with the rear wheels, a driver can maintain the vehiclestationary in the fallback braking mode by depressing the brake pedal soas to supply hydraulic fluid pressure is supplied to all of the wheelsof the vehicle, which is effective to brake the vehicle once it has beenbrought a speed at or near standstill.

A primary braking system may become fully non-functional when theability to provide any hydraulic fluid pressure to the foundation brakesis lost completely. The ability to provide any hydraulic fluid pressureto the foundation brakes may be lost, for example, because of a brakecontroller failure or because of a complete loss of the hydraulicbraking fluid. When the primary braking system is fully non-functional,proceeding from step 350 of method 300 (see FIG. 3), the controller 20may initiate, a transition to a fallback braking mode in which theworking volume of the braking system is reduced such that hydraulicfluid pressure is supplied to fewer than all of the wheels of vehicle10. For example, controller 20 may supply hydraulic fluid to brakingdevices of only two wheels of vehicle 10 (i.e., wheels of a singleaxle), such as front wheels 12 and 14, but not rear wheels 16 and 18.Thus, the braking power of the hydraulic fluid pressure is focused onfront wheels 12 and 14. Parking brake system 60 may be used to brakerear wheels 16 and 18. Further, additional methods of applying dynamicbraking torque may be applied during a transition to a fallback brakingmode.

Turning to FIG. 5, an exemplary method 500 to transition to a fallbackbraking mode once the primary braking system has been determined to befully non-functional at step 350 of method 300 is shown. Method 500includes the same steps 405-445 that are included in method 400 (totransition to a fallback braking mode once the primary braking systemhas been determined to be partially functional and are described above.However, additional steps are included in method 500 so that additionalmethods of applying dynamic braking torque can be applied during atransition to a fallback braking mode.

At step 410 of method 500, information may be received about the amountof force being applied to the brake pedal by the driver. For example,force information can be detected and/or approximated by a pedal travelsensor, a pedal angle sensor, an ABS unit pressure sensor, or any othermethod for sensing a driver's intent for braking, and then communicatedto the brake controller. Similar to method 400, in step 415 of method500, the amount of force being applied to the brake pedal by the drivermay be determined based on the information received in step 410 in orderto determine whether the parking brake system 60 should be actuated tosupply braking energy to the rear wheels. Further, in step 505 of method500, the amount of force being applied to the brake pedal by the drivermay be determined based on the information received in step 410 in orderto determine whether actuate additional methods of supplying dynamicbraking torque to the vehicle. Additional methods of applying dynamiccounter-propulsive torque can include engine braking methods,transmission braking methods, and methods of adding engine loads.

Engine braking methods can include spark retarding and/or engine valveclosure. Spark retarding and engine valve closure inhibit combustionsuch that engine pistons fail to turn the wheels. Engine valve closureincreases the resistance to turning the pistons beyond the normalresistance due to friction.

Transmission braking can be initiated by downshifting a vehicle with anautomatic transmission or actuating a low gear in a vehicle with ashift-by-wire transmission. Shifting to a gear with a lower gear ratioreduces that speed at which the engine can rotate and providescounterpropulsive force by limiting the rotational speed. Transmissionbraking can have increased influence when coupled with engine brakingmethods disclosed above.

Engine loads may be added by engaging the air conditioning of a vehicle,raising the alternator set point voltage, and/or any other loadsoperably connected to belts connected to the engine. Increasing engineloads requires more of the torque generated by the engine to be divertedto provide energy for operating the engine loads instead of providingpropulsive energy to the wheels.

Referring again to the method 500 of FIG. 5, if the amount of forcebeing applied to the brake pedal by the driver is determined to be belowa specified threshold level of force, the method may proceed to step510, in which at least one of the additional methods of applying dynamiccounter-propulsive torque (i.e., engine braking, transmission braking,and adding engine loads) are not actuated to supply braking energy tothe rear wheels of the vehicle. The method may proceed from step 510 tostep 410 to repeatedly determine whether to actuate each of theadditional methods of applying dynamic counter-propulsive torque on aperiodic basis. If the amount of force being applied to the brake pedalby the driver is determined to meet or exceed a specified thresholdlevel of force, the method may proceed to step 515, in which actuationof the one or more of the additional methods of applying dynamic brakingtorque occurs. Each type of the additional methods of applying dynamiccounter-propulsive torque (i.e., engine braking, transmission braking,and adding engine loads) can have a unique specified threshold level offorce that triggers actuation of the method at step 515. In other words,threshold levels of force for each additional method of applying dynamiccounter-propulsive torque may be staggered such that as the amount offorce applied to the brake pedal increases, the amount of additionalmethods of applying dynamic counter-propulsive torque that are actuatedincreases. Actuation of the additional methods of applying dynamiccounter-propulsive torque in this way can provide the driver a morefamiliar brake pedal feel during a transition to a fallback braking modeand smooth out the feel of the overall transition.

Steps 505 through 515 may occur concurrently with or after the workingvolume of the hydraulic braking system is reduced such that hydraulicfluid pressure is supplied to only to the front wheels of the vehicle atstep 405. Also, steps 505 through 515 may occur concurrently with theactuation and modulation of the parking brake system at steps 410through 445. Once the working volume of the hydraulic braking system hasbeen reduced at step 405, the electronic parking brake has been actuatedto supply braking energy to the rear wheels via dynamic clamping in aslip dependent manner at step 445, and all of the additional methods ofapplying dynamic counter-propulsive torque have been actuated at step515, the method may proceed to step 550, in which information may bereceived about whether or not the vehicle is at a standstill may bereceived. For example, vehicle speed and/or standstill information canbe detected or approximated by a speedometer, GPS, a sensor operablyconnected to the power train (e.g., in the transmission or electricmotor drive), or any other method for sensing a vehicle's motion, andthen communicated to the brake controller. In step 555, it may bedetermined whether or not the vehicle is at a standstill based on theinformation received in step 550 in order to bring the braking systeminto the final fallback braking mode.

If it is determined in step 555 that the vehicle is not at a standstill,the method 500 proceeds to step 560, in which the working volume of thehydraulic braking system is maintained at a reduced volume, theelectronic parking brake remains actuated to supply braking energy tothe rear wheels via dynamic clamping in a slip dependent manner, and theadditional methods of applying dynamic counter-propulsive torque remainactuated. The method may proceed from step 560 to step 550 to repeatedlydetermine whether to bring the braking system into the final fallbackbraking mode on a periodic basis. If it is determined that the vehicleis at a standstill, the method may proceed to step 565, in which thebraking system is brought into the final fallback braking mode. To bringthe system into the fallback braking mode the brake controller mayreopen the rear wheel inlet valves and/or a primary pressure supplyvalve in order to increase the working volume of the hydraulic brakingsystem such that any remaining hydraulic fluid pressure is supplied toall of the wheels of the vehicle (i.e., both the front and the rearwheels). Also, in bringing the system into the fallback braking mode,the parking brake may be removed from engagement with the rear wheels.Further, in bringing the system into the fallback braking mode, theadditional methods of applying dynamic braking force may be disengaged.

Another type of failure that can occur in the overall braking system isa failure of the braking devices (e.g., the braking devices (not shown)at the wheels 12, 14, 16, 18 of vehicle 10 in FIGS. 1 and 2). Turning toFIG. 6, an exemplary method 600 may run concurrently with either ofpreviously described methods 400 or 500 to transition to a fallbackbraking mode once the primary braking system has been determined to bepartially functional or fully non-functional functional at steps 340 or350, respectively, of method 300 is shown. Method 600 is applicable forvehicles with a shift-by-wire system. Shift-by-wire vehicles thatincorporate method 600 can apply counter-propulsive torque to the wheelsof the vehicle by shifting the vehicle into reverse or park.

At step 605 of method 600, information may be received about thefunctionality of the braking devices at the wheels may be received. Forexample, braking device functionality information can be detected by abraking device sensor or any other method for the availability of thebraking devices, and then communicated to the controller. If it isdetermined in step 610 that the braking devices at the wheels arefunctional, the method 600 proceeds to step 615, in which the shift toreverse or park is not initiated. The method may proceed from step 615to step 610 to repeatedly determine whether the braking devices at thewheels are functional on a periodic basis. If it is determined that thebraking devices at the wheels are non-functional, the method may proceedto step 620, in which additional aspects are monitored, such as thebrake pedal travel and speed of the vehicle, to determine whether ashift to reverse or park will be initiated during a transition to afallback braking mode.

Once step 620 is reached, where the foundation brakes are indicated tobe non-functional, method 600 may proceed to step 625, in whichinformation about the force being applied to the brake pedal andinformation about the speed of the vehicle may be received. For example,force information can be detected and/or approximated by a pedal travelsensor, a pedal angle sensor, an ABS unit pressure sensor, or any othersuitable method for sensing a driver's intent for braking, and vehiclespeed information can be detected and/or approximated by a speedometer.Both pedal force information and vehicle speed information can thencommunicated to the brake controller. Once the information iscommunicated, method 600 may proceed to step 630. At step 630 the amountof force being applied to the brake pedal by the driver and the speed ofthe vehicle may be determined based on the information received in step625 in order to determine whether to shift the vehicle into reverse orpark, or to delay such a shift. If the amount of force being applied tothe brake pedal by the driver is determined to be below a specifiedthreshold level of force and/or the speed of the vehicle is determinedto be above specified threshold level of speed, then the method mayproceed to step 635, in which use of the shift-by-wire capability of thevehicle to shift the vehicle into reverse or park is delayed. The methodmay proceed from step 635 to step 625 to repeatedly determine whether toshift the vehicle into reverse or park. If both the amount of forcebeing applied to the brake pedal by the driver is determined to meet orexceed a specified threshold level of force and the speed of the vehicleis determined to be at or below a specified threshold level of speed,the method may proceed to step 640, in which the shift-by-wirecapability of the vehicle is used to shift the vehicle into reverse orpark. An exemplary threshold level of force may be an amount required toachieve maximum brake pedal travel possible, although other thresholdlevels of force are contemplated. An exemplary threshold level of speedmay be about 20 kilometers per hour (kph) or less, or about 5 kph orless. In other words, if it is determined that the brake pedal is fullyapplied (i.e., amount of force being applied is sufficient to achievefull pedal travel) and the speed of the vehicle has been sufficientlylowered (e.g., vehicle speed is below about 20 kph or below about 5kph), then the vehicle is shifted into reverse or park. In general,threshold levels of force and/or speed can be calibrated. Once a vehicleis at a standstill (i.e., when the vehicle is determined to be at astandstill at step 460 or 560 of concurrently processing methods 400 or500, respectively), the brake-by-wire system of the vehicle can beshifted out of reverse or park so that the braking system may be broughtto a final fallback mode of braking.

According to an alternative exemplary embodiment where a vehicle can beshifted into reverse, a speed threshold may not be required to shift thevehicle into reverse. According to another exemplary embodiment anembodiment where the vehicle is shifted into park, the park pawl of thevehicle may be applied to shift into park. Where the vehicle is shiftedinto park by applying the park pawl, a speed threshold is required dueto the pawl ratcheting effect.

It is to be understood that the particular examples and embodiments setforth herein are non-limiting, and modifications to structure,dimensions, materials, and methodologies may be made without departingfrom the scope of the present teachings.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the written description and claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the present invention. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the present teachings are approximations, thenumerical values set forth in the specific examples are reported asprecisely as possible. Any numerical value, however, inherently containscertain errors necessarily resulting from the standard deviation foundin their respective testing measurements. Moreover, all ranges disclosedherein are to be understood to encompass any and all sub-ranges subsumedtherein.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the,” include plural referents unlessexpressly and unequivocally limited to one referent. As used herein, theterm “include” and its grammatical variants are intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that can be substituted or added to thelisted items.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the devices and methods ofthe present disclosure without departing from the scope of itsteachings. Other embodiments of the disclosure will be apparent to thoseskilled in the art from consideration of the specification and practiceof the teachings disclosed herein. It is intended that the specificationand embodiment described herein be considered as exemplary only.

What is claimed is:
 1. A method for transitioning a braking system of avehicle between primary and fallback modes, comprising: identifying afailure in functionality of the primary braking system; upon identifyingthe failure, mitigating an abrupt increase in a pedal travel distancerequired to brake or otherwise decelerate the vehicle so as to provide asmooth transition from a primary braking mode to a fallback brakingmode, the mitigating the increase in the pedal travel distancecomprising: initiating a transition to the fallback braking mode;activating at least one of a plurality of transition braking modes; andgradually increasing the pedal travel distance by deactivating at leastone of the at least one previously activated transition braking modes.2. The method of claim 1, wherein a first transition braking mode of theplurality of transition braking modes comprises reducing a workingvolume of a primary braking system so that hydraulic braking energy issupplied to wheels of an axle of a vehicle.
 3. The method of claim 2,wherein a second transition braking mode of the plurality of transitionbraking modes comprises selecting a source of braking energy to besupplied to wheels of an axle of the vehicle by an electronic parkingbrake.
 4. The method of claim 3, wherein selecting a source of brakingenergy includes: supplying braking energy via antilock braking when adetermined speed of the vehicle is above a threshold speed; andsupplying braking energy via dynamic clamping in a slip dependent mannerwhen the determined speed of a vehicle is at or below a threshold speed.5. The method of claim 1, wherein a third transition braking mode of theplurality of transition braking modes comprises actuating at least oneadditional method of applying dynamic counter-propulsive torque to thevehicle selected from the group consisting of an engine braking method,a transmission braking method, and an adding engine load method.
 6. Themethod of claim 5, wherein the engine braking method includes sparkretarding or engine valve closure.
 7. The method of claim 5, whereinwhen the vehicle has an automatic transmission, the transmission brakingmethod includes downshifting the automatic transmission.
 8. The methodof claim 5, wherein when the vehicle has a shift-by-wire transmissionthe transmission braking method includes actuating a lower gear.
 9. Themethod of claim 5, wherein the adding engine load method includesengaging the air conditioning system of the vehicle.
 10. The method ofclaim 5, wherein the adding engine load method includes raising analternator set point voltage of the vehicle.
 11. The method of claim 1,wherein a fourth transition braking mode of the plurality of transitionbraking modes comprises applying counter-propulsive force to the vehicleby using a shift-by-wire system to shift the vehicle into reverse orpark.